1. Field of the Invention
This invention relates to a process for producing an .alpha.-olefin polymer using an improved Ziegler-Natta catalyst.
2. Description of the Prior Art
Recent years have seen a remarkable development of catalysts for obtaining .alpha.-olefin polymers with a high stereoregularity in a high yield. In particular, it is known that the use of a solid catalyst component comprising a carrier mainly composed of magnesium halide compound having supported thereon a transition metal compound as a transition metal component remarkably increases the activity per unit weight of transition metal, and that polymers with a high stereoregularity can be obtained if an electron donor is present upon preparation of the solid catalyst component. Typical processes for preparing such solid catalyst components include pulverizing anhydrous magnesium chloride together with an electron donor and a titanium halide complex (British Pat. No. 1,387,888 to 1,387,890); pulverizing anhydrous magnesium chloride together with an electron donor and a silicon halide compound, and treating the co-pulverized product with titanium tetrahalide (British Pat. No. 1,492,618); and pulverizing anhydrous magnesium chloride together with an electron donor and treating the co-pulverized product with a tetravalent titanium halide compound (Japanese Patent Publication No. 50037/77).
From the standpoint of simplifying or eliminating the step of removing a catalyst residue remaining in the product .alpha.-olefin polymer, it goes without saying that the lower the content of the catalyst residue in the product .alpha.-olefin polymer is, the better. For this reason, a high polymerization activity per unit weight of transition metal has been eagerly desired as well as a high content of transition metal compound in the solid catalyst.
From this standpoint, the above-described processes are still unsatisfactory and several improved processes have been proposed. For example, there are processes of using, as one constituent of the solid catalyst component, an aluminum halide-ether complex as described in Japanese Patent Application (OPI) No. 73991/77 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"), an alkoxyaluminum compound as described in West German patent application (OLS) No. 2,701,647, or an alcohol or phenol as described in West German Patent Application (OLS) No. 2,708,588. In these processes, however, catalyst performance is still not fully satisfactory. An increase in the transition metal content in the solid catalyst usually results in reduction of polymerization activity per unit weight of transition metal, thus the advantages of an improvement in the transition metal content in the solid catalyst are cancelled or results in reduction of stereoregularity of the .alpha.-olefin polymer (or isotactic index, hereinafter abbreviated as I.I.), thus practical values are spoiled.
The inventors have already disclosed in West German Patent Application (OLS) No. 2,818,642 a process of preparing a catalyst by pulverizing a solid reaction product of magnesium hydroxychloride and ethylaluminum dichloride together with ethyl benzoate and a siloxyaluminum compound, and reacting the co-pulverized product with titanium tetrachloride. However, the step of preparing the carrier in this process is complicated and thus the process is economically disadvantageous. Furthermore, the reproducibility of polymerization performance of the produced solid catalyst is poor.